NEW YORK (Reuters Health) - Eating fish rich in omega-3 fatty acids doesn't appear to stave off the blues in women, U.S. researchers have found.
Their study, published in the American Journal of Clinical Nutrition, adds to the conflicting evidence on the benefits of fish oil, which some research has hinted might help certain people with depression.
"We know that omega-3s are important in brain function," study researcher Dr. Alberto Ascherio, a nutrition expert at the Harvard School of Public Health in Boston, told Reuters Health.
"We approached this work thinking that when it comes to preventing depression, it's conceivable that you are what you eat," he said.
But the researchers' findings didn't bear out that prediction.
The team followed nearly 55,000 nurses over 10 years. All the women, between 50 and 77 years old, were free of depression when the study began in 1996.
Over the next decade, five percent of them eventually developed clinical depression. But the risk was the same regardless of how much DHA and EPA -- two omega-3 fatty acids -- women got from eating fish.
Fish rich in omega-3s include salmon, trout, sardines and herring.
The researchers did find preliminary signs that a plant-based omega-3 called alpha-linolenic acid could play a role in mood.
For every increase of half a gram in daily intake of the substance --common in walnuts and canola oil, for instance -- there was an 18-percent reduction in the risk of depression.
A study like the current one can't prove cause-and-effect, and Ascherio said the area needs further research before any recommendations can be made.
His team also examined omega-6 fatty acids, but was unable to come up with conclusive findings on its impact on depression. Omega-6s are found in refined vegetable oils and are ubiquitous in snack foods, sweets and fast foods.
Depression strikes twice as many women as men, with one in five U.S. women experiencing the problem at some point.
Dr. Teodore Postolache, who directs the mood and anxiety program at the University of Maryland School of Medicine in Baltimore, told Reuters Health he is not yet ready to give up on fish oil.
"There are inherent limitations on studies about depression, including determining with certainty what exactly depression is for patients," Postolache said.
Using data from nurses, for example, can skew results because nurses are more educated in matters of health and diet than the general population.
"If groups who may have underlying deficits in fish oil were studied, like lower socioeconomic groups, we might have seen a more powerful effect of the omega-3s in preventing depression," he said.
He also noted that the study excluded women who had previous episodes of depression, although this group is one of "the most important targets for intervention because they are at high risk for a repeat episode."
He called for more research on animals and in broader swaths of the population.
SOURCE: bit.ly/kmB4rn American Journal of Clinical Nutrition, April 2011.